This invention relates generally to the field of rigid disc drives, and more particularly, but not by way of limitation, to a glide test head assembly for use in evaluating surface characteristics of a magnetic recording disc.
Data storage devices of the type known as xe2x80x9cWinchesterxe2x80x9d or xe2x80x9chardxe2x80x9d disc drives are well known in the industry. Such disc drives record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless direct current (dc) spindle motor. In disc drives of the current generation, spindle motors rotate the discs at speeds of up to 10,000 revolutions per minute (rpm).
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent to the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator housing opposite to the coil, the actuator housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted.
When current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator housing rotates, the heads are moved radially across the data tracks along an arcuate path.
As the physical size of disc drives has decreased historically, the physical size of many of the disc drive components has also decreased to accommodate this size reduction. Similarly, the density of the data recorded on the magnetic media has been greatly increased. In order to accomplish this increase in data density, significant improvements in both the recording heads and recording media have been made.
For instance, the first rigid disc drives used in personal computers had a data capacity of only 10 megabytes, and were in the format commonly referred to in the industry as the xe2x80x9cfull height, 5xc2xcxe2x80x3xe2x80x9d format. Disc drives of the current generation typically have a data capacity several gigabytes in a 3xc2xdxe2x80x3 package which is only one fourth the size of the full height, 5xc2xcxe2x80x3 format or less. Even smaller standard physical disc drive package formats, such as 2xc2xdxe2x80x3 and 1.8xe2x80x3, have been established. In order for these smaller envelope standards to gain market acceptance, ever greater recording densities must be achieved.
The recording heads used in disc drives have evolved from monolithic inductive heads to composite inductive heads (without and with metal-in-gap technology) to thin-film heads fabricated using semi-conductor deposition techniques to the current generation of thin-film heads incorporating inductive write and magneto-resistive (MR) read elements. This technology path was necessitated by the need to continuously reduce the size of the gap in the head used to record and recover data, since such a gap size reduction was needed to reduce the size of the individual bit domain and allow greater recording density.
Since the reduction in gap size also meant that the head had to be closer to the recording medium, the quest for increased data density also lead to a parallel evolution in the technology of the recording medium. The earliest Winchester disc drives included discs coated with xe2x80x9cparticulatexe2x80x9d recording layers. That is, small particles of ferrous oxide were suspended in a non-magnetic adhesive and applied to the disc substrate. With such discs, the size of the magnetic domain required to record a flux transition was clearly limited by the average size of the oxide particles and how closely these oxide particles were spaced within the adhesive matrix. The smoothness and flatness of the disc surface was also similarly limited. However, as the size of contemporary head gaps allowed data recording and retrieval with a head flying height of about 3,000 Angstroms (xc3x85), corresponding to about 300xc3x9710xe2x88x929 meters (300 nm) or about 12xc3x9710xe2x88x926 inches (12 xcexcin), the surface characteristics of the discs were adequate for the times.
Disc drives of the current generation incorporate heads that fly at nominal heights of around 380 A (about 38 nm or 1.5 xcexcin), with efforts underway to reduce this flying height to below 250 A (25 nm or 1.0 xcexcin). Clearly, the surface characteristics of the discs must be much more closely controlled to accommodate such reduced flying heights.
In current disc drive manufacturing environments, it is common to subject each disc to component level testing before it is assembled into a disc drive. One type of disc test is referred to as a xe2x80x9cglidexe2x80x9d test, which is used as a go/no-go test for surface defects or asperities, or excessive surface roughness. A glide test typically employs a precision spin stand and a specially configured glide test head including a piezo-electric sensing element, usually comprised of lead-zirconium-titanate (PbZrTi3), also commonly known as a xe2x80x9cpzt glide test head.xe2x80x9d The glide test is performed with the pzt glide test head flown at approximately half the flying height at which the operational read/write head will nominally fly in the finished disc drive product. If the glide test is completed without contact between the pzt glide test head and any surface defects, then the disc is passed on the assumption that there will be no contact between the operational heads and the discs during normal operation.
On the other hand, if contact occurs between the pzt glide test head and a defect on the disc surface, the disc is subjected to a burnishing process in an attempt to remove or reduce the size of the offending media surface defect. The disc is retested and, if the burnishing operation was successful, the disc is approved for incorporation into a disc drive. Any disc which fails to pass the glide test after burnishing is scrapped.
A variant of the glide test, often used by disc media manufacturers, is sometimes referred to as a xe2x80x9cglide avalanchexe2x80x9d or GA test. In GA testing, a pzt glide test head is first flown at a greater than normal flying height above the disc surface. This initial increased flying height is commonly achieved by rotating the disc under test at a greater than normal speed, thus increasing the linear velocity between the disc and the test head, and increasing the strength and thickness of the air bearing supporting the test head above the disc surface.
The rotational speed of the disc under test is then gradually reduced until contact between the test head and disc occurs, at which point the current flying height is recorded. Correlation of a series of such test sequences at varying radii on the disc can be used by the disc media manufacturer as an indication of overall disc surface characteristics.
It is also common practice in the industry to provide a textured xe2x80x9clanding zonexe2x80x9d on the disc surface, on which the read/write head of the disc drive will come to rest during xe2x80x9cpower-offxe2x80x9d or xe2x80x9csleepxe2x80x9d conditions. Since the glide avalanche test simulates the loss of power to rotate the disc, the glide avalanche test is also frequently used by design engineers developing textured landing zones to study the landing characteristics of head assemblies on various types of landing zone textures.
With new disc drive products being developed to operate with operational head flying heights of about 250 A (25 nm or 1.0 xcexcin), disc drive manufacturers have the need to perform glide tests using pzt glide test heads which can operate in a stable manner at flying heights of about 125 A (13 nm or 0.5 xcexcin), a requirement which is difficult to achieve using existing glide test head technology.
Glide test heads typically include positive-pressure air bearing structures, and it is difficult to produce a positive-pressure air bearing glide test head which is stable at a flying height of 125 A (13 nm or 0.5 xcexcin), as fly height variance (three-sigma standard deviation) can be as high as 113 A (11 nm or 0.45 xcexcin). The instability of current generations of glide test heads at such flying heights results in head/disc contacts that are caused by this flying instability instead of reflecting true glide test results, and leads to the possibility of erroneous over-rejection of discs that are, in actuality, suitable for use.
A need clearly exists, therefore, for an improved glide test head and test system capable of reliably glide testing recording discs at substantially reduced flying heights.
The present invention is directed to an apparatus and method for evaluating surface characteristics of a recording disc prior to incorporation into a disc drive.
In accordance with preferred embodiments, a glide test system is provided which includes a glide test head supportable over the disc. The glide test head has a negative-pressure air bearing slider and a contact sensor which outputs a signal when the glide test head contacts a feature of the disc surface.
The glide test head and the disc are characterized as opposing plates of a variable capacitor with a dielectric layer therebetween including at least a layer of air supporting the glide test head. A voltage source, operably coupled to the glide test head and the disc, applies a fly height control voltage across the capacitor to adjust the fly height of the glide test head.
The disc surface preferably comprises a data region configured to magnetically store data as the disc is rotated and a texturized landing zone configured to support the disc drive read/write head when the disc is stopped. The voltage source accordingly applies a first fly height control voltage to maintain the glide test head at a first glide distance over the data region and a second fly height control voltage to maintain the glide test head at a second, greater glide distance over the landing zone.
These and various other features as well as advantages which characterize the present invention as claimed below will be apparent from a reading of the following detailed description and a review of the associated drawings.